By Paul McLearyPaul McLeary is Foreign Policy’s senior reporter covering the U.S. Defense Department and national security issues. He joined the Washington office in 2015 after working for Defense News, where he was also on the Pentagon beat, and covered stories relating to Pentagon spending and the defense industry. While there, and in a previous incarnation as a New York-based reporter, Paul embedded with U.S. Army and Marine Corps units in Iraq and Afghanistan to cover ground combat operations, where he got inside a secretive drone program being run out of Bagram air base. He has also traveled with the U.S. Navy, covered NATO meetings in Europe with the chairman of the Joint Chiefs of Staff, and stalked major international arms shows in Paris and London.

Russia warned Monday that any U.S. or coalition aircraft flying west of the Euphrates River in Syria will be tracked by Russian warplanes and anti-aircraft batteries, a swift reaction to Sunday’s shoot-down of a Syrian Su-22 bomber by an American F-18.

It is unclear if the Russians have the capability to track the dozens of sorties flown by U.S. and coalition aircraft over Syria on any given day, but the threat further raises tensions as U.S.-backed Arab and Kurdish fighters press on the Islamic State’s stronghold of Raqqa, and American forces increasingly tangle with Iranian-backed militias in Syria’s south.

Russia is a staunch ally of the Syrian regime of Bashar al-Assad, along with Iran, and the downing of the Syrian warplane by a U.S. fighter jet represented yet another escalation in an increasingly tense situation unfolding in southeastern Syria. With Islamic State losing territory, forces loyal to the Syrian regime and Iranian-backed militia are increasingly butting heads with troops aligned with the United States.

As part of its protest against the shooting down of the Syrian jet — which the Russian Ministry of Defense called a “flagrant violation of international law, in addition to being actual military aggression against the Syrian Arab Republic” — Moscow also said it was shutting off the hotline maintained by U.S. and Russian military officers in the region, where each side provides warnings about impending air operations in Syria.

U.S. defense officials said on Monday the hotline remains open. Speaking in Washington on Monday afternoon, Chairman of the Joint Chiefs Gen. Joseph Dunford said the two sides discussed the matter as recently as Monday morning, and urged patience as the two sides continued to discuss operations in Syria.

“I’m confident that we are still communicating between our operations center and the Russian Federation’s operations center,” Dunford told an audience at the National Press Club. “I’m also confident that our forces have the capability to take care of themselves.”

Russia briefly shut down the line in April, after U.S. ships fired 59 Tomahawk cruise missiles at a Syrian air base in response to a chemical weapons attack on civilians launched from the base.

“The Russian Federation has indicated that their purpose in Syria, like ours, is to defeat ISIS, and we’ll see if that’s true here in the coming hours,” Dunford said.

“We will continue to conduct air operations throughout Syria,” despite the Russian rhetoric, spokesman for the U.S.-led coalition Col. Ryan Dillon told Foreign Policy Monday. The shoot-down was in “accordance with the rules of engagement and international law,” he said.

Dunford backed that up during his appearance Monday, saying the shoot-down was legal under the 2001 authorization Congress passed for the U.S. military to strike al Qaeda and its offshoots. Since U.S. forces are targeting the Islamic State in Syria, they are covered under that blanket protection.

The Russian statement was careful not to promise to shoot down coalition aircraft, but warned that any coalition aircraft “will be followed by Russian ground-based air defense and air defense aircraft as air targets.”

The shoot-down of the Su-22 was the first time an American plane shot another manned aircraft down since an incident in Bosnia in the late 1990s. Last week, an F-15 shot down a Iranian-made drone that had attacked U.S. commandos and a group of anti-ISIS Syrian fighters on patrol in southern Syria, near the Iraqi border.

Russia has deployed its S-300 and S-400 air defense systems to bases in western Syria, but it is doubtful they could engage aircraft as far away as Raqqa. Michael Kofman, a research scientist at CNA Corporation told FP that due to the presence of mountains between the Russian coastal batteries and the rest of the country, “they probably can only see only at really high altitudes, and even then it’s doubtful they can see out that far east,” to target aircraft near Raqqa.

The incident began on Sunday after pro-regime forces — a blanket term the Pentagon attaches to the groups of Iranian-backed Hezbollah fighters, Iraqi Shiite militias and other groups fighting alongside Syrian government forces — attacked a U.S.-backed Syrian Democratic Forces unit near the city of Taqba, west of Raqqa. The assault came despite what one U.S. defense official told FP was an agreement between the local SDF commander and the Syrian commander not to attack one another in the specified area.

The United States’ involvement in the six-year old Syrian conflict is getting more complicated. The downing of the Syrian jet, and the subsequent Russian warnings, come as American forces and their allies have grown increasingly entangled with Iranian-backed forces in Syria. As the Islamic State’s hold on territory shrinks, Iran has pushed to gain a foothold over as much territory as possible to keep lines open from its border with Iraq, all the way through to Damascus, and on to Lebanon.

U.S. warplanes have bombed Iranian-backed Hezbollah fighters three times over the past month, after they moved too close to a U.S. garrison at al-Tanf near the Iraqi border.

Col. Dillon said that the U.S. is tracking the pro-regime forces as they continue to move east from the Taqba area toward Deir Ezzor province that borders Iraq. U.S. commanders have said much of the Islamic State’s leadership has fled Raqqa for the villages along the Euphrates River Valley in the province, and they expect the city of Maydan, in the valley, to be another large battle.

On Sunday, Iran fired six ballistic missiles at Islamic State targets in Deir Ezzor in response to the recent attack on the Iranian parliament building and a shrine in Tehran.

The Trump administration has been engaged in an internal debate about how to respond to the presence of Iranian-backed militia in southeastern Syria, with some White House officials pushing for a more aggressive approach that would prevent Iran and its proxies from securing the Iran-Syria border area.